Management of Isolated Local Failures Following Stereotactic Body Radiation Therapy for Low to Intermediate Risk Prostate Cancer

Background: Stereotactic body radiation therapy (SBRT) is a safe and effective treatment option for patients with low to intermediate risk prostate cancer. SBRT delivers high biologically effective doses resulting in very low PSA nadirs. Strategies to diagnose and confirm salvageable isolated local failures are needed. This study aims to determine the incidence and approach to management of isolated local failures (ILF) after SBRT in a large single institution cohort. Method: All patients with low or intermediate risk localized prostate cancer treated with SBRT at Georgetown University Hospital were eligible for this study. Treatment was delivered using robotic SBRT with doses of 35-36.25 Gy in five fractions. ILF were diagnosed using multiparametric MRI and/or biopsy prompted by concerning PSA rise after achieving long-term nadir. Patient’s characteristics were extracted from a prospective institutional quality of life trial (IRB#2009-510). Choice of salvage therapy and post-salvage PSA were ascertained on subsequent follow up and chart review. Results: Between December 2008 to August 2018, 998 men with low to intermediate risk prostate cancer were eligible for inclusion in this analysis. Twenty-four patients (low risk, n=5; intermediate risk, n=19) were found to have ILF within the prostate on either MRI (n=19) and/or biopsy (n=20). Median pre-treatment PSA was 7.55ng/ml. Median time to diagnosis of ILF was 72 months (24-110 months) with median PSA at the time of ILF of 2.8ng/ml (0.7-33ng/ml). Median PSA doubling time was 17 months (5-47 months). Thirteen patients with biopsy proven ILF proceeded with salvage therapy (cryotherapy n=12, HIFU n =1). Of 12 patients who underwent cryotherapy, 7 had a post-treatment PSA of <0.1ng/ml. One patient experienced a urethral-cutaneous fistula. Conclusion: The incidence of isolated local recurrence is rare in our series. Diagnosis and management of isolated local failures post-SBRT continues to evolve in the era of successful salvage therapy. Our report suggests an important role for early utilization of MRI and confirmatory biopsy at relatively low PSA levels and long PSA doubling time. Additionally, undetectable PSA post-salvage therapy supports early treatment after identifying isolated local failures. Larger trials are needed for refinement of patient selection and most appropriate salvage modality.